Millimeter microwave generator



April 1969 T. E. HASTY 3,436,680

MILLIMETER MICROWAVE GENERATOR Filed June 16, 1967 PRIOR ART 3 m IO ;;III2 35,13

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INVENTOR TURNER E. HASTY ATTORNEY United States Patent 3,436,680MILLIMETER MICROWAVE GENERATOR Turner E. Hasty, Dallas, Tex., assignorto Texas Instruments Incorporated, Dallas, Tex., a corporation ofDelaware Filed June 16, 1967, Ser. No. 646,716 Int. Cl. H03b 5/12, 5/32US. Cl. 331-107 6 Claims ABSTRACT OF THE DISCLOSURE The classic mode ofoperation of a bulk negative resistance device is the transit timefrequency mode described by J. B. Gunn in the early 1960s The Gunn modeof operation is achieved by placing a bulk negative device in anelectrical circuit which presents a low impedance load to the device andbiases it above the critical voltage. The frequency of oscillations of abulk device operated in the Gunn mode wtih a low impedance load circuitis approximately equal to the recirocal of the electron transit timethrough the sample. If the low impedance load is replaced by a resonantcircuit, the frequency of oscillations in the bulk device can be variedby simply changing the resonant frequency of the load.

Recent investigations have established that a new mode of operationknown as the limited space charge accumulation (LSA) can be effected bythe use of a proper load and bias for the device. Devices operating inthe LSA mode have true negative resistance characteristics and areuseful as millimeter energy sources having at least tens of milliwattsavailable continuous power and possibly kilowatts of pulse power, andoperating at efiiciencies in the 10% to range. The LSA mode of operationhas been achieved in samples of n-type GaAs.

A bulk negative resistance device will operate in the LSA mode if theresonant frequency of its associated circuitry is in the correct rangefor the doping level. This frequency is considerably higher than(typically four times) the natural transit time frequency of the bulkdevice. Because a negative resistance device is not self-starting in theLSA mode, a high Q resonant circuit having a very large resistance(typically one hundred times) as compared with the low field resistanceof the bulk negative device has been used during the initial oscillationperiod. The biasing voltage connected to the bulk device is three orfour times the threshold voltage when operating in the LSA mode, and theradio frequency appearing across the device has a voltage swing greatenough to drive the device below the threshold value during part of eachcycle. When operating in the LSA mode, the radio frequency electricfield changes from below the threshold value to a value more than twicethe threshold field so quickly that the space charge distributionassociated with a high field domain does not have time to form.

In accordance with the present invention, the frequency of oscillationsin a bulk negative resistance device is initiated by an energy waveinicdent thereon. To produce a wave capable of transmitting tens ofmilliwatts continuous power, a plurality of bulk devices are arranged inseries each receiving an energy wave of low power from the precedingdevice and transmitting an energy wave of high power to a subsequentdevice. The initial wave is generated by a bulk negative resistancedevice operating in the Gunn mode followed by a harmonic generator and afilter for suppressing the fundamental and all harmonics up to a presetlevel. The Gunn mode oscillator, the filter, and the plurality of bulkdevices are mounted in a microwave circuit which can be in the form of astripline integrated circuit. Thus, there is provided a millimetermicrowave generator producing a relatively high power output without theuse of high Q circuits.

Another object of the invention is to provide a millimeter microwavesource that is self-starting when fully loaded.

Still another object of the invention is to provide a millimetermicrowave source completely formed on a stripline integrated circuit.

Yet another object of the invention is to provide a millimeter microwavesource having bulk negative resistance devices operated in the LSA mode.

A more complete understanding of the invention and its advantages willbe apparent from the specification and claims, and from the accompanyingdrawings illustrative of the invention.

Referring to the drawings:

FIGURE 1 is a schematic of an equivalent electrical circuit showing amillimeter microwave generator of the prior art;

FIGURE 2 is a plot of an electric field across a bulk negativeresistance device in the transit time mode and the LSA mode;

FIGURE 3 is a plot of drift velocity vs. electric field in a negativeresistance device and the radio frequency driving voltage when operatedin the limited space charge accumulation mode; and

FIGURE 4 is a schematic of a microwave millimeter source in accordancewith this invention.

Although not necessarily limited thereto, this descrip tion will proceedto describe a millimeter microwave source using gallium arsenide (GaAs)as bulk negative resistance devices. Other materials, such as many ofthe IIIV semiconductor materials, that exhibit negative resistancecharacteristics can be used.

The element of primary importance in the operation of the presentmillimeter microwave generator is the bulk negative resistance device.The transit time frequency mode of such a device was described by Gunnin his work with gallium arsenide (GaAs) bulk negative resistancedevices. Prior to Gunns work, it had been determined that GaAs exhibitednegative resistance characteristics when an electric field of greaterthan 3,000 volts per centimeter was applied. It was also establishedprior to Gunns experiments, that the electric field across a crystal ofGaAs in the negative resistance region is not uniform but has a domainstructure. As a result of this domain structure in the electric fielddistribution, it was felt that GaAs negative resistance devices werelimited in usefulness. Gunn discovered that the high field domain wasnot stationary at one point in the crystal, but actually traveled acrossthe crystal at approximately the drift velocity of an electron. Althoughthe Gunn phenomena may result in the production of a useful source ofmicrowave energy at frequencies between 6 gHz. and 30 gHz., it does notappear that significant power can be obtained from waves at higherfreqencies.

In prior art millimeter microwave generators, the bulk negativeresistance device is connected in series with a high Q parallel tunedcircuit and a load resistor to suppress the field domain. Referring toFIGURE 1, there is shown schematically a circuit representing anequivalent wave guide wherein a bulk negative resistance device 10 isoperated in series with a tuned circuit having a high Q and including acapacitor 11 and an inductor 12. A load resistor 13 is connected acrossthe tuned circuit and a choke coil 15 in series with a D.C. voltagesource 14, shown schematically as a battery, provides the necessarybias. The circuit shown is known as a LSA oscillator, that is, anoscillator having a bulk negative resistance device operating in alimited space charge accumulation mode. If the radio frequency voltageacross the diode 10 can be made sufficiently high, there results amodified form of Gunn operation in which field domain does not form andthe frequency of oscillations can be determined by the resonantfrequency of the tuned circuit. The characteristics of the LSA mode are:(l) the frequency of operation is higher than the reciprocal of thecarrier transit time (2) the frequency of oscillation is determined bythe resonant frequency of the tuned circuit, and (3) the power outputand efiiciency are at least equal to but usually greater than the poweroutput and efiiciency of a Gunn oscillator.

In operation, although the LSA mode is inherently efficient because theentire device is operated in a negative state over most of the electricfield cycle, there are limitations to its usefulness when operated in asystem of the type shown in FIGURE 1. This is primarily because of thedifficulties in causing a bulk negative resistance device to operate inthe limited space charge accumulation mode. Since the LSA mode ofoscillation is not self-starting, it is necessary for a large radiofrequency electric field to build up across the crystal before it breaksinto oscillations. To start the LSA oscillator of FIGURE 1, the device10 is connected to the DC. bias 14 and made to generate an energy waveat its natural transit time frequency. This wave includes a fundamentalfrequency and its harmonics and is directed through a wave guide to thetuned circuit of capacitor 11 and inductor 12. The tuned circuit isadjusted to resonant at a particular harmonic of the fundamentalfrequency of the energy wave and all harmonics up to a preferred level.The preferred harmonic is fed back to the device 10 thereby causing itto operate at the resonant frequency of the tuned circuit instead of itsnatural frequency. Thus, the starting energy is generated by operatingthe crystal in a limited space charge mode in a lightly loaded conditionand mounted in a reasonably high Q cavity. Since the optimum load for asystem such as shown in FIGURE 1 is different in the starting mode thanin the operating mode, it is necessary to adjust the load 13 to achievemaximum power after oscillations have commenced by means of a tuningslug. Care must be exercised when adjusting the tuning slug to preventoverloading the diode 10 thereby causing oscillations to cease.

In the LSA mode, the negative resistance device 10 operated with a large(8,000 v./cm.), high frequency electric field across its terminals. Theamplitude of the electric field is such that the terminal voltage acrossthe dewere is driven below threshold during the negative half cycle ofthe field, and the frequency of the field is high enough so that itsperiod is short compared to the dielectric rela ration time of thedevice when biased in the negatrve resistance region. The electric fielddistribution vs. distance for the transit time frequency mode ofoperation described by Gunn and the limited space charge accumulationmode are plotted in FIGURE 2. The electric field distribution is plottedon the vertical axis versus the distance across the crystal on thehorizontal axis. The critical field strength above which negativeresistance characteristics are exhibited is identified on the verticalaxis by Fr: and shown as a dotted line. The domain structure (shownsolid), represents the operation of a negative resistance device in thetransit time frequency mode and may be several times larger thanactually shown. The dotted line curve represents a true negativeresistance device such as the device 10 operating in the LSA mode.Because the period of the radio frequency field generated by the tunedcircuit is short, the domain does not have time to form and truenegative resistance characteristics result.

Referring to FIGURE 2, there is shown the drift velocity of theelectrons in a negative resistance device versus the electric fieldstrength across its terminals. The radio frequency voltage generatedacross the device is illustrated by the curve 16. With reference to thesystem of FIGURE 1, the DC. bias source 14 biases the device 10 at thepoint 17 on the horizontal axis. Before generation of the radiofrequency curve 16, the device 19 operates in the Gunn mode as describedpreviously, and its field strength curve would be similar to that shownin FIGURE 2. After the radio frequency curve 16 has been generated, thedevice 19 is driven below the threshold level during part of each cycle.This high frequency electric field rises from below the thresholdvoltage to a value more than three times the threshold value and dropsagain so quickly that the space charge distribution associated with thehigh field domain does not have time to form. Instead, there is only anaccumulation layer formed at the cathode contact and the rest of thedevice is in a negative resistance region. When the electric field dropsbelow the threshold value, the space charge which has accumulated duringthe positive half cycle of the radio frequency field must decay. Thisdecay can only be achieved if the dielectric relaxation of the material,at an electric field below threshold, is large compared with the periodof oscillation of the electric field. The dielectric relaxation time ofa material is defined by the relation -L n lul where e is the dielectricconstant, it is the carrier concentration, e is the electronic chargeand p. is the electron mobility.

In accordance with this invention, a Gunn oscillator operating at itsnatural frequency triggers oscillations in the first crystal of a chainall operating in a limited space charge accumulation mode. Referring toFIGURE 4, there is shown a millimeter microwave generator including abulk negative resistance device 18 mounted in a wave guide 19 andconnected to an appropriate DC bias source. not shown, for operation inthe transit time frequency mode. The wave generated by the device 18 istransmitted through the wave guide 19 to a harmonic generator 21 of adiode configuration. The output of the harmonic generator 21 is filteredby appropriate wave guide filtering techniques to eliminate thefundamental frequency generated by the device 18 and all harmonics up toa preferred value. Downstream of the wave guide filtering system thereis mounted a plurality of negative resistance devices 22, 23, and 24biased above their threshold levels by DC. sources, not shown, foroperation in the LSA mode and mounted in the wave guide 19. Any numberof negative resistance devices could be so mounted. The negativeresistance devices 22, 23, and 24 are spaced in the wave guide 19 anintegral number of half wavelengths apart. The number of integral halfwavelengths between adjoining resistance devices is determined bymechanical considerations and not to achieve any particular operatingcharacteristics.

In operation, the negative resistance device 18 is biased to operate inthe transit time frequency mode and generate low power oscillations atits natural frequency. The electric field across the device 18 has adomain structure as was described with reference to FIGURE 2. As such,it is not operating as a true negative resistance device. The low poweroutput of device 18 is transmitted to the diode 21 which generates anonlinear function having the fundamental and harmonics of the signalincident thereon. In accordance with standard Wave guide filteringtechniques, the nonlinear function generated by the diode 21 is filteredto suppress the fundamental and all harmonics up to a preferred level.The preferred harmonic is transmitted through the wave guide 19 andincident upon the negative resistance device 22. As mentionedpreviously, a negative resistance device operates at about 8,000 voltsper centimeter in the LSA mode. Since the output of the device 18 is arelatively weak signal, the first negative resistance device 22 must bethin enough to insure operation in an 8,000 volt per centimeter electricfield. The negative resistance device 22 amplifies the preferredharmonic which is transmitted to and supplies the radio frequency fieldto the second negative resistance device 23. The energy wave incidentupon the device 23 corresponds to the wave 16 shown in FIGURE 3. Assuch, it drives the device 23 below threshold once during each cycle andtrue negative resistance characteristics are achieved such as shown bythe dotted curve of FIGURE 2. Since the energy wave incident upon thedevice 23 is amplified by the device 22, the active length of the secondnegative resistance device may be two times that of the first device andstill operate with a radio frequency field in excess of 8,000 volts/cm.The output of the negative resistance device 23 is an amplified energywave of the preferred harmonic which is incident upon and functions asthe radio frequency field to the third negative resistance device 24.The negative resistance device 24 again amplifies the preferred harmonicwhich is transmitted through the wave guide to another negativeresistance device or to a load circuit. Any number of negativeresistance devices may be arranged serially to generate a signal of thedesired power level.

A millimeter microwave generator of the type shown in FIGURE 4 isself-starting and can be connected to the desired load during thestart-up operation. In addition, the system of FIGURE 4 does not requirea high Q circuit during start up.

In a typical embodiment of the system shown in FIG- URE 4, the Gunnoscillator consisted of the GaAs device five microns thick andgenerating a frequency of gHz. The harmonic generator consists of adiode mounted in a wave guide filter to suppress the fundamental, thesecond, and the third harmonics of the wave generated by the harmonicgenerator and pass the fourth and all higher harmonics. The mostsignificant frequency passed by the filter system is the fourth harmonichaving a frequency of 80 gHz. which is the frequency of the outputsignal. The 80 gHz. signal from the filter is the radio frequencyelectric field to the first of three LSA operated devices. The first LSAoperated device is a two to five micron thick crystal of GaAs. Thesecond LSA operated device and the third LSA operated device are alsoGaAs crystals having a thickness of ten microns and twenty microns,respectively.

Although the above description exemplified a millimeter microwavegenerator employing wave guide construction, such a system will beparticularly useful in integrated microwave stripline circuits. One ofthe advantages of this invention is that LSA operation is possible inrelatively low Q circuits whereas in the conventional LSA oscillator, asshown in FIGURE 1, a high Q circuit is required for starting. For anintegrated microwave stripline the entire circuit is fabricated on amonolithic slice of semi-insulating GaAs about 0.004 inch thick. Aground plane is formed over the bottom surface of the substrate by ametallized film, which may be D.C. isolated from the GaAs substrate byan insulating layer of silicon dioxide. The substrate provides thedielectric for a microstrip transmission line.

The Gunn oscillator is comprised of a heavily doped n-type region formedin the upper surface of the substrate and a more lightly doped n-typeregion formed within the heavily doped region. In general, the highresistivity GaAs in the lightly doped region exhibits afrequency-sensitive negative-resistance effect under the influence of ahigh electric field established between the low resistivity heavilydoped region and a metallized contact deposited on the higherresistivity region. In practice, the low resistivity region and thehigher resistivity region are formed by successive epitaxial depositionsin a pit etched in the surface of the semi-insulating GaAs substrate.

A semi-insulating GaAs substrate has been defined as being a form ofgallium arsenide which has a resistivity greater than 10ohm-centimeters. This material can be prepared by adding various dopingelements, which produce deep levels, to the gallium arsenide duringcrystal preparation. High resistivity GaAs can also be obtained withoutcompensation with the doping elements which produce the deep levels byachieving high purity.

A load impedance connected to the Gunn oscillator is designed tomaintain oscillation and provide adequate power output. Provisions aremade for D0. biasing the higher resistivity region and connecting oneterminal to ground. A one-half wavelength open ended microstrip stub isadequate to provide an RF ground.

The harmonic generator is typically a Schottky barrier diode of GaAs inwhich a rectifying junction is formed between a metallic film and arelatively lightly doped ntype GaAs layer. For example, a heavily doped,low resistivity GaAs region is formed with four relatively highresistivity n-type GaAs expitaxial regions formed t erein.

The output of the harmonic generator is coupled to the input of astandard stripline design. A complete description of a stripline filter,harmonic generator, and Gunn oscillator is given in the copending US.application of Tom M. Hyltin, Ser. No. 606,097, filed Dec. 30, 1966 andassigned to the same assignee.

The LSA operated negative resistance devices are fabricated inessentially the same manner as the Gunn oscillator. Again, each iscomprised of a heavily doped n-type region formed in the upper surfaceof the semi-insulating GaAs substrate and a more lightly doped n-typeregion formed within the heavily doped region. Typical thicknesses ofthe various LSA operated devices are five microns for the first device,ten microns for the second device, and twenty microns for the thirddevice. Since the LSA mode of oscillation in GaAs requires a radiofrequency field of approximately 8,000 volts per centimeter to start,the first LSA mode device is relatively thin to insure that the RFvoltage from the Gunn oscillator is sufficient to start oscillations.And since each LSA mode operated device amplifies its input signal,thicker devices are located at integral number of half wavelengths away.

While several embodiments of the invention, together with modificationsthereof, have been described in detail herein and shown in theaccompanying drawings, it will be evident that various furthermodifications are possible in the arrangement and construction of itscomponents without departing from the scope of the invention.

I claim:

1. A millimeter microwave generator comprising:

a GaAs crystal operated in the transit time frequency mode forgenerating a low power frequency Wave, means for generating a harmonicof said frequency wave, and

a plurality of GaAs crystals operated in the limited space chargeaccumulation mode generating oscillations at the harmonic frequency andpositioned in a serial arrangement, the first of said plurality of GaAscrystals located in the field of said harmonic wave and each of theother of said crystals located in the wave generated by the precedingcrystal displaced from said harmonic generating means.

2. A millimeter microwave generator as set forth in claim 1 wherein theactive length of each of said plurality of GaAs crystals is two timesthe active length of the preceding crystal displaced from said harmonicgenerating means.

3. A millimeter microwave generator as set forth in claim 2 including amicrowave guide in which said first GaAs crystal, said harmonicgenerating means, and said plurality of crystals are mounted.

7 8 4. A millimeter microwave generator comprising: 6. A millimetermicrowave generator as set forth in a GaAs crystal operated in thetransit time frequency claim 5 wherein said plurality of GaAs crystalsare 10- mode for generating a low power frequency wave, cated anintegral number of half wavelengths from each a harmonic generatorresponsive to said wave for generother.

ating the harmonics thereof,

means for filtering the fundamental and all harmonics References Citedbelow a predetermined level, and UNITED STATES PATENTS a plurality ofGaAs crystals operated in the limited 3,189,843 6/19 5 B k 331 1 7 spacecharge accumulation mode positioned in :1 3,320,550 5/19 7 G -l 331 1 7serial arrangement, the first of said plurality of cry- 10 3,339,1538/1967 H kki 331-5 stals responsive to the lowest harmonic wave passed3,354,403 11/1967 c ll 331-55 by said filter and each subsequent crystalresponsive 3,378,739 4/1968 Gerlflch 331 56 to the wave generated by theprevious crystal in said serial arrangement. JOHN KOMINSKI, PrimaryExaminer.

S. A millimeter microwave generator as set forth in 15 claim 4 whereinsaid first GaAs crystal, said harmonic generator, said filter, and saidplurality of GaAs crystals 331 56 are part of a stripline microwavecircuit.

